Seasonal benthic nitrogen cycling in a temperate shelf sea: the Celtic Sea

• Published in: Biogeochemistry 2017-09, Vol.135 (1/2), p.103-119
• Main Authors: Kitidis, V., Tait, K., Nunes, J., Brown, I., Woodward, E. M. S., Harris, C., Sabadel, A. J. M., Sivyer, D. B., Silburn, B., Kröger, S.
• Format: Article
• Language: English
• Subjects: Ammonium; Ammonium compounds; Biogeosciences; Blooms; Cohesive sediments; Continental shelves; Correlation; Cruises; Cycles; Denitrification; Earth and Environmental Science; Earth Sciences; Ecosystems; Efflux; Environmental Chemistry; Fluxes; Gene expression; Hydrazine; Life Sciences; Mineral nutrients; Mud; Nitrification; Nitrogen; Nitrogen cycle; Nutrients; Ocean floor; Organic matter; ORIGINAL PAPERS; Oxidation; Oxidoreductase; Oxidoreductases; Oxygen consumption; Phytoplankton; Pore water; Porosity; Profiles; Removal; Seasons; Sediments; Summer; Water column
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-017-0311-3

We undertook a seasonal study of benthic N-cycling on the Celtic Sea continental shelf in 2015, augmented by an earlier cruise in 2014. Two cruises in 2015 were centred before and after the Spring phytoplankton bloom and a further cruise was carried out in late summer. Five sites covering the mud to sand continuum were visited on all cruises, where we determined ammonium-oxidation, anammox and denitrification rates, expression of anammox and denitrification genes, N-nutrient fluxes and sediment porewater profiles of N-nutrients. Highest process rates were found during the post-bloom and late summer periods. The Celtic Sea was overwhelmingly a source of inorganic-N to the overlying water column. The efflux of nitrate was controlled by the magnitude of ammonium-oxidation. The latter accounted for 10–16% of total Oxygen consumption in cohesive sediments and 35–56% in sandy sediments. Ammonium oxidation rates in the range of 0.001–2.288 mmol m⁻² days⁻¹ were inversely correlated with sediment porosity and positively correlated with organic matter content (OM) which together explained 66% of the variance in rates. N-removal was dominated by anammox (0.003–0.636 mmol m⁻² days⁻¹), rather than denitrification (0.000–0.034 mmol m⁻² days⁻¹). This finding was supported by the corresponding gene expression data. The expression of hydrazine oxidoreductase (anammox) was significantly correlated with anammox and total N-removal rates. Anammox was positively correlated with porosity and OM, whilst denitrification was correlated with OM. The N-requirement of these processes was largely met through nitrification (ammonium-oxidation) rather than influx from the overlying water column. We estimated that N-removal via denitrification and anammox removed 6–9% of the organic-N deposited at the sea-floor from the overlying water column. The Celtic Sea system was thereby losing N which must be replenished on an annual basis in order to sustain productivity.